I want to model radiation inside a concentric tube in tube heat exchanger. The hot fluid is wrapped with an "air gap" to insulate it. The air gap is wrapped with the cold fluid. I currently think the S2S solver is appropriate but I am also considering the DO solver. I am having trouble with my view factors and clusters in the S2S. I think that my material properties are incorrect for the DO.

Any help would be appreciated. Curtis Schreiber

If the model isn't too large (i.e., too many surfaces) and the "air gap" does not have participating media, consider using DTRM. There aren't as many properties as is needed for DO.